The present invention relates to liquid containment systems such as membrane roof systems, pond liners and storage tank liners. More particularly the present invention relates to field splices or lap seams in liquid containment systems and the method of forming these field splices.
Membrane type liquid containment systems include a membrane which is fluid tight, covering the entire system to prevent liquid from penetrating, for example through the roof deck and into the building or into the ground from a pond. Membranes which are about fifty feet wide and two to three hundred feet in length can be formed in a factory. However, due to manufacturing difficulties the width (50 feet) cannot be substantially increased. Where a roof, pond or the like is larger than fifty feet wide, it is necessary to form the membrane by adhering two or more factory produced membranes together at overlapped edges or seams. These are typically referred to as field splices.
A field splice is formed by bonding together overlapping edges of adjacent membranes, i.e., the lower surface of an edge of a first membrane is bonded to the upper surface of an edge of a second membrane. The field splice, which is about three to twelve inches wide, generally includes a thin layer of an internal splicing adhesive or cement which provides a strong secure bond between the two sheets. In the past, water sensitive splicing cements were prevalent. However, non-water sensitive cements are also used. In order to protect the splicing cement from moisture and to seal any capillaries or other gaps in the splice, a lap sealant is applied to the exterior of the field splice at the outer exposed edge of the upper membrane.
Forming these field splices requires a multi-step process. First the upper and lower edges of the two membranes to be spliced are cleaned to remove talc or other building site foreign material. The splicing cement is then applied to about six inches of both the top and bottom edges of the two membranes. This is allowed to dry for a period of about 5 to 30 minutes during which time the solvent evaporates and the adhesive develops body or strength. This is required to provide sufficient bond tack or greater strength to ensure bonding. Otherwise, the edges could become separated before the adhesive sets and there could be a field splice failure. After the adhesive has partially dried, the upper edge is pressed against the lower edge. A lap sealant is then applied to the exterior exposed side edge of this field splice.
This method of forming a field splice although widely accepted in the elastomeric membrane industry has several disadvantages. The first and primary disadvantage is that the lap sealant is exposed to the elements. Field splices are typically designed to last at least fifteen years. The lap sealants which can withstand weather for fifteen years are expensive.
Further, the lap sealant may be separated from the field splice if stepped on or subjected to force. Because of the location, the lap sealant simply lying against the field splice may not have the ability to form a good physical bond. Should the lap sealant fail, a leak is likely to develop where the splicing cement did not form a perfect seal.
There are various reasons why the splicing cement will not form a perfect seal. When large elastomeric membranes are formed at the factory, they are formed by splicing together through heat and vulcanization even smaller membranes. (For example, twenty (10'.times.50') membranes can be bonded together to form one membrane which is 200 feet long and 50 feet wide). Where two of the 10'.times.50 membranes are vulcanized together, there is an enlarged cross-section or factory seam. Because such a thin film of splicing cement is applied in the field, it frequently does not fill in the gaps created where the field splice includes a factory seam. This is particularly true if two factory seams are overlapped.
Also, when these field splices are formed, pressure is required to force the two edges together to make a good bond. Any irregularity in the supporting structure will prevent a uniform application of pressure across the entire seam. This can cause gaps. Further, if the membranes are not perfectly aligned, an edge can pucker and create a gap. Finally, simply because such a thin film of cement is used, capillaries can form which will allow the passage of water.
In addition to these problems with the integrity of the field splice, the very method of forming the field splice presents disadvantages. Specifically it is very time consuming. The splicing cement which is applied to the outer edges of the membranes must be allowed to partially dry before the upper membrane can be pressed down against the lower membrane. Otherwise, the adhesive will not have sufficient bond tack to initially hold the membranes together. This means the applicator forming the field splice must wait while the cement is drying. This is a significant waste of manpower.
Systems have been designed to overcome these problems. One such system, for example, is disclosed in Kelly U.S. Pat. No. 4,192,196 which describes a complicated roofing system using multiple layers of splicing cement and exterior superficial protective layers. This method is complicated and expensive and therefore unacceptable for current liquid containment systems.
Other systems include snap fitting sheets such as Simpson et al, U.S. Pat. No. 4,296,582. Such interlocking systems are excessively expensive to manufacture and accordingly unsuitable for use in current roofing systems. Further, Cook U.S. Pat. No. 3,716,434 discloses a thermoplastic bonding material which includes application of multiple beads of hot melt. This method calls for application of excessive amounts of hot melt and requires application of a molten hot adhesive to a thermoplastic material. Hot melt adhesives set too quickly and are unsuitable for most roofing applications. To form a bond, the edges must be pressed against the hot melt before it cools. To date, this has proved to be impractical for field application.